This is fairly solid except how they would be k-selected would they only have very rare hatching events? Then there is the matter of this species probably being most effective in the topics filling a niche that would struggle for variety and providing new forms of aquaculture. However I don't think that these shell-men are a going to be only wise old pacifists they would also be excellent merchants especially due to being able to access pearls, corals (dyes, and body scrubbers), and ambergris just to name a few. Another reason they would be excellent merchants is their long life span and if they can remember well enough, they can figure out oceanic pattern early and begin transoceanic trade routes earlier than other species or civs. Probably the hardest bit would be fine work like mechanical objects or gem cutting due to webbed extremities and they might also struggle with smelting iron before figuring out leverage tools to make them but cooper and bronze instruments would be doable and maybe preferable due to resistance to rusting.